(1) Field of the Invention
The present invention relates to a display panel driver and a driving method for display devices represented by liquid crystal displays and plasma displays.
(2) Description of the Related Art
There have been known liquid crystal display devices that display an image on a liquid crystal display panel in accordance with a video signal supplied from an external apparatus (see Japanese Patent Application Laid-open 2004-151222). In a liquid crystal display device of this kind, the video signal supplied externally is converted into digital signals, then stored temporarily in a memory. Then, data is read out from the memory in response to a predetermined timing signal and supplied to a driver circuit of the liquid crystal display panel. The timing signal required to drive the liquid crystal display panel is generated based on a horizontal reference signal (a signal that specifies the display period of one scan line or horizontal line that constitutes an image frame) and a vertical reference signal (a signal that specifies the vertical period or the display period of one image frame) which are generated internally.
In the aforementioned display device, however, if the frequency of the vertical reference signal varies, a discontinuity takes place in the drive timing signal for driving the liquid crystal display panel. As a result, the number of horizontal lines (scan lines) during one vertical period changes, causing the problem that a stable image display becomes impossible.
As an example where the frequency of the vertical reference signal changes, a case as follows can be considered.
When an external image signal (especially for motion pictures such as a video signal) is stored into a memory (e.g., field memory) and data is read out from the memory upon a predetermined timing signal, writing data into the memory is performed based on a vertical synchronizing signal externally supplied. In contrast, reading data from the memory is preformed based on a vertical reference signal which is generated based on an internal clock which is asynchronous with the external vertical synchronizing signal. Accordingly, there occur some cases where the timing of reading data outpaces the timing of writing data, other cases where the timing of reading data is outpaced by the timing of writing data. In such a case, if there is a time lag between the write frame and the read frame (that is, in a case where a motion picture is handled), there occurs a problem that the display image is shifted sidewards. To deal with this, there is a need for suppressing the occurrence of a sideward image shift by adjusting the timing of writing data into the memory and the timing of reading data from the memory. An adjustment method has been known, in which the vertical reference signal, based on which timing signal for data reading and timing signal for driving the liquid crystal panel are determined, is switched into a signal which is synchronized with the external video signal, i.e., the vertical synchronizing signal which is externally supplied. In this case, a change in the frequency of the vertical reference signal occurs when the vertical reference signal generated within is switched into the vertical synchronizing signal from without.
Further, while the external vertical synchronizing signal is being used as a vertical reference signal in the aforementioned adjustment method, functions such as fast forward play, rewinding play, etc., are performed on a VTR (video tape-recorder) as an external apparatus, the frequency of the synchronizing signal (vertical synchronizing signal) from the VTR changes, with the result that the frequency of the vertical reference signal changes.